Field of the Invention
The present invention relates to reducing the optical peak power of laser pulses in the temporal domain and, optionally, to homogenizing the beam power distribution in a spatial domain. This peak power reduction and homogenization system may use curved mirrors, beam splitters, wave plates, and prisms to generate an optimized pulse repetition-rate multiplier with a flat-top spatial power distribution profile. The present invention is particularly useful in semiconductor inspection and metrology systems.
Related Art
The illumination needs for inspection and metrology are generally best met by continuous wave (CW) light sources. A CW light source has a constant power level, which allows for images or data to be acquired continuously. However, at many wavelengths of interest, particularly ultraviolet (UV) wavelengths, CW light sources of sufficient radiance (power per unit area per unit solid angle) are not available, are expensive or are unreliable. If a pulse laser is the only available, or cost-effective, light source with sufficient time-averaged radiance at the wavelength of interest, then using a laser with a high repetition rate and wide pulse width is best. The higher the pulse repetition rate, the lower the instantaneous peak power per pulse for the same time-averaged power level. The lower peak power of the laser pulses results in less damage to the optics and to the sample or wafer being measured, as most damage mechanisms are non-linear and depend more strongly on peak power rather than on average power.
In inspection and metrology applications, an additional advantage of an increased repetition rate is that more pulses are collected per data acquisition or per pixel leading to better averaging of the pulse-to-pulse variations and improved signal-to-noise ratios. Furthermore, for a rapidly moving sample, a higher pulse rate may lead to a better sampling of the sample position as a function of time, as the distance moved between each pulse is smaller.
The repetition rate of a laser subsystem can be increased by improving the laser medium, the pump system, and/or its driving electronics. Unfortunately, modifying a UV laser that is already operating at a predetermined repetition rate can require a significant investment of time and money to improve one or more of its constituent elements, and may only improve the repetition rate by a small increment. Furthermore increasing the repetition rate of the fundamental laser in a UV laser reduces the peak power of the fundamental. This reduces the efficiency of the frequency conversion (which is necessarily a non-linear process) and so makes it harder to generate high average UV power levels.
In many inspection applications, a flat or uniform, rather than Gaussian, illumination profile is desired. Spatially uniform illumination on the sample results in a more uniform signal-to-noise ratio across the illuminated area and a higher dynamic range compared with non-uniform illumination. Although incoherent light sources may be able to more readily generate uniform illumination than the Gaussian profile of a laser, such light sources have much broader bandwidth (complicating the optical design because of chromatic aberration) and lower power density (reducing signal-to-noise ratios) than a laser can provide. One known way to achieve an approximately flat profile from a Gaussian laser beam is to crop off the Gaussian tails and only use the central region (close to peak) of the beam. This method is simple to apply; however, if a reasonably flat profile is required a large fraction of the laser power is cropped off and wasted. For example, if the maximum intensity variation in the illumination is required to be about 10%, then about 65% of the power is wasted, and a 20% variation requires wasting approximately 50% of the power.
Therefore, a need arises for a practical, inexpensive technique to improve the repetition rate of a UV laser that operates on the output of the laser. Furthermore it would be advantageous if the optical subsystem that increases the repetition rate can be compact so that it can readily be incorporated into a system without taking up a lot of space. Still furthermore there is a need for a repetition rate multiplier that can generate an approximately flat output profile while adding no, or few, additional components to the repetition rate multiplier, thus saving space and minimizing optical power losses.